A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. The strokes are typically denoted as compression and power strokes. In a two-stroke cycle, opposed-piston (“OP2S”) engine two pistons are disposed crown-to-crown in the bore of a cylinder for reciprocating movement in opposing directions. The cylinder has longitudinally-spaced inlet and exhaust ports formed in the cylinder sidewall near respective ends of the cylinder. Each of the opposed pistons controls a respective one of the ports, opening the port as it moves toward a bottom dead center (BDC) location during a power stroke (also called an expansion stroke), and closing the port as it moves from BDC toward a top dead center (TDC) location during a compression stroke. One of the ports provides passage for the products of combustion out of the bore, the other serves to admit pressurized air into the bore; these are respectively termed the “exhaust” and “intake” ports.
OP2S engines typically operate according to the compression-ignition principle. During a compression stroke, turbulent pressurized air (“charge air”) enters the bore of a cylinder through the intake port and is compressed between the end surfaces of the two pistons as they move from BDC toward TDC. Fuel injected between the approaching piston end surfaces mixes with the turbulent air. Near TDC of the pistons, the air/fuel mixture is ignited by the heat of the compressed air, and combustion follows.
In a uniflow-scavenged OP2S engine, near the end of a power stroke, charge air entering a cylinder through the intake port displaces exhaust gas flowing out of the cylinder through the exhaust port. Thus gas flows through the cylinder in one direction (“uniflow”)—from intake port to exhaust port. A continuous positive pressure differential must exist from the intake ports to the exhaust ports of the engine in order to maintain the desired unidirectional flow of gas in the cylinders. In many cases, this need is met by an engine air handling system that includes a fast acting, positive-displacement pump, upstream from the engine's intake ports. This role is typically filled by a supercharger (or blower) which is driven mechanically by an engine crankshaft, by direct connection, as with a belt or a chain. The air handling system may include a turbocharger, powered by exhaust energy, and including a compressor that is placed ahead (upstream) of the supercharger in order to increase the density of intake air fed into the supercharger. Depending on design requirements, a uniflow-scavenged, OP2S engine may include either or both of a mechanically-driven supercharger and an exhaust-driven turbocharger.
Operation of an OP2S engine subjects it to demands for torque which may result from acceleration, deceleration, transmission shifts, switching accessories (like air conditioning) on or off, pulling a trailer, climbing a hill, and so on.
A demand for torque generates a requirement to increase the quantity of fuel provided to the engine in order to raise the level of energy released by combustion. This requires a concurrent provision of additional air to burn the additional fuel. A uniflow-scavenged, OP2S engine equipped with electronically-controlled air and fuel handling systems may produce smoke, or produce other hydrocarbon emissions, or experience misfiring in response to a torque demand if the response time of the air handling system lags that of the fuel handling system. For example, in diesel applications of an OP2S engine, without timely provision of additional air in response to a torque demand the increased level of diesel fuel may be incompletely burned, which can result in the transport of particulate matter comprising unburned fuel droplets, incompletely burned fuel, and soot in the exhaust stream, thereby creating smoke.
Therefore, it is desirable to limit the consequences of incomplete combustion due to air handling lag in response to a torque demand during operation of a uniflow-scavenged, OP2S engine. Consequently, during a torque demand, a limiting factor for the engine's operation may be defined by the delay in providing sufficient air to burn the additional fuel provided in response to the demand.
Provision of fuel and air in the engine are governed by an engine control mechanization that senses various engine operating parameters and regulates the flow of gasses (air and exhaust) through the engine and the injection of fuel into the engine. It is particularly desirable that the engine control mechanization be able to limit the additional fuel quantity provided in response to a torque demand while the air handling system increases the airflow to a level sufficient to support full combustion at the requested torque level.